This section provides background information related to the present disclosure which is not necessarily prior art.
In view of increased consumer interest in four-wheel drive vehicles, a plethora of power transfer systems are currently being utilized in vehicular applications for selectively directing rotary tractive power (i.e., drive torque) from the powertrain to all four wheels of the vehicle. In many power transfer systems, a transfer case is used for delivering drive torque from the powertrain to one or both of the front and rear drivelines. Some conventional transfer cases are equipped with a mode clutch, typically a positive-locking type of dog clutch, that can be selectively actuated via operation of a mode shift mechanism to shift between a two-wheel drive mode and a part-time (i.e. locked) four-wheel drive mode. In addition, many transfer cases also include a two-speed reduction unit and a range clutch which can be selectively actuated via operation of a range shift mechanism for shifting between four-wheel high-range and low-range drive modes.
It is also known to use “on-demand” power transfer systems for automatically distributing the drive torque generated by the powertrain between the front and rear drivelines, without any input or action on the part of the vehicle operator, when traction is lost at either the front or rear wheels. Modernly, it is known to incorporate the “on-demand” feature into a transfer case by replacing the mechanically-actuated mode clutch with a multi-plate clutch assembly and a power-operated clutch actuator that is interactively associated with an electronic control system and a plurality of vehicle sensors. Such transfer cases configured to permit electronic control over the drive torque distribution between the front and rear drivelines are commonly referred to as “active” transfer cases. During normal road conditions, the multi-plate clutch assembly is typically maintained in a release condition such that drive torque is only delivered to the rear wheels. However, when the sensors detect a low traction condition, the power-operated clutch actuator is operated for engaging the multi-plate clutch assembly to deliver drive torque automatically to the front wheels. Moreover, the amount of drive torque transferred through the multi-plate clutch assembly to the front wheels can be varied as a function of specific vehicle operating characteristics and/or road conditions, as detected by the sensors. This adaptive clutch control system can also be used in full-time transfer cases to automatically bias the torque distribution ratio across an interaxle differential.
A majority of current active transfer cases include a rear output shaft interconnecting the transmission output to the rear driveline, a front output shaft interconnected to the front driveline, a transfer assembly driven by the front output shaft, and a power-operated clutch assembly arranged to selectively/automatically couple the transfer assembly to the rear output shaft for transmitting drive torque to the front driveline. Typically, the transfer assembly includes a first sprocket rotatably supported on the rear output shaft, a second sprocket fixed for rotation with the front output shaft, and a chain encircling and drivingly interconnecting the first sprocket for rotation with the second sprocket. The clutch assembly and various components of the power-operated clutch actuator are typically disposed to surround the rear output shaft and function to selectively/automatically couple the first sprocket to the rear output shaft.
Such active transfer cases also require lubrication of the clutch assembly and other rotary components. A sump of lubricant is maintained in a lower portion of the transfer case so as to typically submerge at least the second sprocket. A passive lubrication system utilizes lubricant splashed throughout the transfer case upon rotation of the sprockets to lubricate the rotary components and to cool the clutch assembly. As alternative, a shaft-driven gear or gerotor lube pump can be operably associated with the rear output shaft to pump lubricant from the sump and distribute such lubricant in response to rotation of the rear output shaft. Finally, an actively-controlled or “on-demand” lube pump can be installed with the transfer case to provide optimal lubricant flow in a manner that is independent of the rotational characteristics of the rear output shaft.
In the past, the vehicle ride height and suspension configuration of many trucks and sport utility vehicles provided sufficient packaging volume for such traditional active transfer cases equipped with a pair of offset output shafts. However, in view of increased demand for smaller four-wheel drive vehicles, the packaging volume allocated to the powertrain and the transfer case has been greatly reduced. To accommodate reduced packaging space, commonly-owned U.S. Pat. No. 8,316,738 discloses an active transfer case having a traditional rear output shaft and clutch assembly configuration in association with a beveloid gearset type of transfer assembly and an angulated front output shaft. Alternatively, some active transfer cases have been developed which position the clutch assembly and power-operated clutch actuator on the front output shaft as shown, for example, in U.S. Pat. No. 8,157,072.
While such alternative transfer case configurations attempt to address the need for reduced packaging requirements, a need still exists to advance the technology and structure of active transfer cases to provide enhanced configurations that improve upon the prior art.